Unlike a conventional programmable logic device (PLD), an in-system programmable logic device (ISPLD) can be reprogrammed in place, i.e. without removal from the system in which it is deployed. ISPLDs are therefore especially suited for implementing dynamically reconfigurable circuits. The method of reprogramming a programmable logic device in place is known as in-system programming (ISP). Some ISPLDs, such as those available from Lattice Semiconductor Corporation, Hillsboro, Oreg., can be reprogrammed using an operating voltage power supply, rather than a high programming voltage, as in the case of many PLDs. U.S. Pat. No. 4,855,954, entitled "In-system Programmable Logic Device with Four Dedicated Terminals," by J.E. Turner et al., assigned to Lattice Semiconductor Corporation, which is also tile assignee of the present Application, discusses ISPLD technology. U.S. Pat. No. 4,855,954 is hereby incorporated by reference in its entirety.
FIG. 1 shows an idealized pin-out of an ISPLD. As shown in FIG. 1, tile ISPLD comprises a number of input-only pins (I.sub.1, I.sub.2, . . . , I.sub.n), a number of programmable input/output pins (I/O.sub.1, I/O.sub.2, . . . , I/O.sub.m), power (VCC) and ground (GND) pins, and a set of ISP pins (SDI, SDO, SCLK, and mode). During operation, if tile ISPLD is to be reprogrammed, the signal associated with the SCLK pin is activated to place the ISPLD into the ISP mode. Once in the ISP mode, a state machine having numerous states takes over the control of the programming activities. The new program is input serially into the program memory of an ISPLD over the serial input pin SDI. The rate of serial input is 1-bit per clock period A clock signal is provided on pin SCLK when the ISP mode is entered. Each ISPLD can also provide on its output pin SDO data received from its serial input pin SDI. Thus, a number of ISPLDs can be "daisy-chained" together by tying the serial input pin SDI of an ISPLD to the serial output pin SDO of another ISPLD. Any ISPLD in the daisy chain can be reprogrammed by shifting in the new program at the serial input pin SDI of the first ISPLD in the daisy chain, and through every ISPLD in the daisy chain ahead in the daisy chain of the ISPLD to be programmed.
Because the pins of an integrated circuit package are considered a scarce resource, minimizing the number of pins dedicated for reprogramming purpose maximizes the number of pins available to the operation for which the ISPLD is deployed. Thus, it is highly desirable to have an ISPLD having very few pins dedicated to reprogramming purpose.